The present invention relates generally to displays and, more particularly, to backlighting systems for displays.
Backlighting an electronic display is a common need for many industries. For example, in the aviation and space industry, the backlit liquid crystal display (LCD) offers display luminance efficiency, contrast ratio and display viewing angles comparable to the once commonly used cathode ray tube (CRT). In addition, unlike CRTs, backlit LCDs provide a compact design with low power requirements, thus making the backlit LCD particularly suited for avionics displays.
Typically, the LCD is backlit using a fluorescent discharge lamp in which light is generated by an electric discharge in a gaseous medium. A conventional fluorescent lamp configured for backlighting a display includes a serpentine fluorescent lamp tube positioned within an interior region of a lamp housing called the backlight cavity. Filaments are mounted within free end portions of the lamp tube. Alternating current (AC) power is provided to the filaments through leads from a power supply. The lamp tube is charged with a mixture of mercury vapor and noble gas and the inner surface of the lamp tube is coated with phosphor.
When the fluorescent lamp is turned on, an electric field inside the lamp tube is produced which ionizes the noble gas. Free electrons become accelerated by the electric field and collide with the mercury atoms. As a result, some mercury atoms become excited to a higher energy state without being ionized. As the excited mercury atoms fall back from the higher energy state, they emit photons, predominately ultraviolet (UV) photons. These UV photons interact with the phosphor on the inner surface of the lamp tube to generate visible light.
The intensity of the visible light generated by the fluorescent lamp depends on the mercury vapor partial pressure in the lamp tube. At a mercury pressure less than the optimum mercury pressure, the light intensity of the fluorescent lamp is less than maximum because the mercury atoms produce fewer UV photons. At a mercury pressure greater than the optimum mercury pressure, the light intensity of the lamp is also less than maximum because so many mercury atoms tend to collide with the UV photons generated by other mercury atoms. Some of these UV photons fail to reach the phosphor coated inner surface and therefore do not generate visible light.
Nonetheless, many manufacturers fill the lamp tube with excess mercury so as to extend the light-output life for several years. As the lamp is burning, the mercury inside the lamp tends to be absorbed into the phosphor lining. The lost mercury is replenished from the excess mercury vapor stored in the lamp. If surplus mercury vapor is released into the lamp, however, the lamp performance diminishes. Therefore, it is desirable to maintain a reservoir within the lamp tube that holds the excess mercury until it is needed.
The mercury vapor pressure increases with the temperature of the coldest location (commonly known as xe2x80x9cthe cold spotxe2x80x9d) inside the lamp tube. The cold spot serves as a point for the excess mercury to coagulate (i.e., the cooler the spot, the greater the attraction of mercury). For many avionics applications, the optimal cold spot temperature for the most favorable mercury pressure within the lamp tube is approximately 55xc2x0 C. To insure that the visible light output of the fluorescent lamp is at a maximum with the least amount of power consumption, it is desirable to regulate the cold spot temperature of the lamp tube to maintain the optimal cold spot temperature.
One known method of regulating the cold spot temperature of the lamp tube is by a thermoelectric cooler (TEC). The typical TEC combines a metal heat sink, a resistive heater, and a diode array. A piece of copper or similar metal is fitted against the foot of the lamp body to form a xe2x80x9ccold shoe.xe2x80x9d The metal extends to the resistive heater and the diode array consisting of a number of individual diodes. A direct current is applied to the TEC, which causes one side to heat up, and the side near the lamp to cool down. This method is an effective way of accelerating the natural heat sinking process.
The TEC usually adequately regulates the cold spot temperature. Nonetheless, the diode arrays tend to be extremely fragile. The diode array should be rugged enough to avoid cracking and fracturing under vibrational loads to which aircraft and spacecraft are commonly subjected, which increases the cost of such arrays. Further, the display should be configured to avoid forces applied to the rigid metal of the cold shoe that is attached to the fragile lamp which could damage the TEC and the lamp. In addition, the TEC design requires additional electronics that tend to occupy display space and increase costs. Further, a significant amount of power may be needed to drive the TEC cooling element.
U.S. Pat. No. 5,808,418, issued Sep. 15, 1998 to Pitman et al., discloses replacing the TEC with a cylindrical glass tube connected to the lamp body. Referring to FIG. 1, a first portion 100 of the tube is exposed to the internal gas pressure of a lamp body 160. A second portion 110 extends outside the housing 120 (backplate) and has a closed end 130. A heating wire 140 is wrapped around the second portion 110 of the tube and controlled by a power supply (not shown). A temperature sensor 150 is mounted on the first portion 100 of the glass tube and coupled to the power supply (not shown).
In operation, the cylindrical tube cools the lamp body 160 by positioning the second portion 130 in cooler air outside the interior of the display. Beyond the backplate 120, outside air circulates, typically from small holes in the airplane fuselage. The extended portion 110 of the tube is cooled by the outside air and thus defines a cold spot for the lamp. The temperature sensor 150 monitors the temperature of the tube near the lamp. If the temperature is below the optimal cold spot temperature range, the sensor 150 energizes the power supply (not shown) so as to deliver power to the heater wire 140. The sensor 150 continually monitors the temperature of the tube 100 and controls, in a feedback loop, the operation of the power supply (not shown) to the heater wire 140.
In another embodiment of the Pitman system, illustrated in FIG. 2, the cylindrical glass tube is replaced by a tin plated copper post 200 having cooling fins 210 attached to the extended portion 220. The post 200 is attached to the fragile lamp body 160 by a thermally conductive silicone adhesive 230. In operation, the copper post behaves substantially identical to the glass cylindrical tube of FIG. 1.
The Pitman system alleviates the need for a TEC, but remains prone to some of the disadvantages associated with the TEC. In particular, the glass cylindrical tube is extremely fragile. Unlike the TEC, the glass tube is open to the internal gases within the fluorescent lamp. Damage to the glass tube necessarily damages the lamp because the glass tube is an integral part of the original lamp body complete with internal lamp gases. If the glass tube breaks while in operation (i.e., in aircraft flight), the entire lamp and the whole display system would be rendered inoperable. While the copper post embodiment may be more resistant to breakage, the rigidity of the post could break the lamp if enough force is applied to the post. Accordingly, the display systems are typically subject to design constraints to minimize potential breakage.
The present invention overcomes the problems outlined above and provides for an improved backlighting system for displays and method for regulating the cold spot temperature of a fluorescent lamp. The system comprises a light emitting enclosure having a defined cold spot. A duct disposed through a backplate is connected to a coolant fluid source at one end and exposed to the cold spot at a second end. Coolant fluid may be allowed to pass by the cold spot.
In an exemplary embodiment, a cold spot regulation system includes an interface housing positioned adjacent to the cold spot and two ducts connected to the interface housing. An intake duct includes an intake end configured to receive a coolant fluid flow and an exhaust duct configured to release the coolant fluid flow. The system may include an inexpensive shock-resilient material that can withstand the vibrations that are common to the aircraft cockpit.
In one embodiment, the system comprises a heating mechanism contiguous with the cold spot mechanism. The heating mechanism may be controlled by a power supply that receives commands from a control circuit. The system may further include a temperature sensor suitably located to monitor the temperature of the cold spot. Temperature readings are supplied to the control circuit. The control circuit energizes the power supply to the heating mechanism as needed to reach an optimum operating temperature.
In yet another embodiment, the heating mechanism comprises a conductive wire wrapped around the light emitting enclosure near the cold spot. In still another embodiment, the heating mechanism comprises a thin film resistive heater that is adhered to the enclosure. The heating mechanism may further comprise an airflow regulation device. The device is designed to open and close the end of the intake duct suitably configured to receive a coolant fluid flow. In operation, the temperature sensor monitors the cold spot temperature and through a control circuit increases or reduces the amount of coolant flow reaching the cold spot.
In still another embodiment, the end of the intake duct that is suitably configured to receive a fluid flow is constricted to form a Venturi tube.